1. Field of the Invention
The present invention relates to a fluid pressure control valve and particularly to a fluid pressure control valve which is optimally applied to a pressure intensifying valve for an automotive anti-skid brake control system.
2. Description of the Prior Art
As is generally known, an anti-skid brake control system is often employed in large luxury cars and high grade vehicles, for preventing brakes from locking road wheels during quick braking or during braking on a low frictional road surface so as to provide maximum effective braking and consequently to reduce a braking distance. The anti-skid brake control system permits normal application of brakes by alternately holding, reducing, holding, and intensifying the wheel-cylinder pressure, such that braking can be held to just below the point at which a skid would start to develop. In order to accomplish the above three modes, namely a pressure reducing mode, a pressure holding mode, and a pressure intensifying mode, the anti-skid brake control system in general includes a pressure intensifying valve arranged in a main brake fluid line interconnecting a master cylinder and a wheel cylinder and a pressure reducing valve arranged in a brake fluid reflux line which is arranged in parallel with the main brake fluid line. The pressure intensifying valve is generally comprised of a normally open type electromagnetic switching valve serving to establish fluid communication between the outlet port of the master cylinder and the inlet and outlet port of the wheel cylinder through the main brake fluid passage and to block the same fluid communication. On the other hand, the pressure reducing valve is generally comprised of a normally closed type electromagnetic switching valve serving to establish fluid communication between the outlet port of the master cylinder and the port of wheel cylinder through the reflux line and to block the same fluid communication. One such normally open type electromagnetic switching valve applicable to a pressure intensifying valve for an anti-skid brake control system has been disclosed in Japanese Patent First Publication (Tokkai) Heisei 3-41278. The prior art electromagnetic switching valve comprises a valve casing, an electromagnetic solenoid attached to the valve casing in a manner which surrounds the outer periphery of the casing, a substantially cylindrical inner yoke disposed in the casing, a plunger slidably enclosed in the yoke, and a poppet valve axially extending from one end of the plunger for opening and closing an opening (inlet port) which is bored in the yoke to introduce the master-cylinder pressure into the valve, and a return spring operably disposed in the yoke for normally biasing the plunger in a direction wherein the poppet valve is opened. The opening of the valve is closed when the solenoid is energized, while the opening is opened when the solenoid is de-energized. In such conventional normally open type electromagnetic switching valves, the inlet port of the valve is rapidly opened as soon as the solenoid is de-energized. In case that such conventional electromagnetic switching valve is utilized as a pressure intensifying valve for an anti-skid brake control system, there is a possibility of a rapid communication between the master cylinder having a relatively high master-cylinder pressure and the wheel cylinder having a relatively low wheel-cylinder pressure, when the operational mode of the anti-skid brake control system is shifted from a pressure holding mode wherein the pressure reducing valve and the pressure intensifying valve are both closed to a pressure intensifying mode wherein the pressure reducing valve is kept closed and the pressure intensifying valve is switched from a fully closed state to a fully open state. This results in a rapid pressure-drop in the master cylinder and causes surge pressure or pulsation of the brake fluid in the wheel cylinder. The driver feels uncomfortable by kickback in the brake pedal and fluid noise created due to surge. Shift from the pressure reducing mode to the pressure intensifying mode results in the same problem.